1. Field of the Invention
This invention relates to an exposure control circuit apparatus, and more particularly to an exposure control circuit apparatus for detecting an average luminance signal of an image pick-up signal by signal charges stored at a plurality of pixels of a solid-state image pick-up device to apply, to the solid-state image pick-up device, a shutter pulse for allowing respective pixels of the solid-state image pick-up device to discharge signal charges on the basis of the signal level of the detected average luminance signal to thereby control the exposure time at respective pixels of the solid-state image pick-up device.
2. Description of the Related Art
A solid-state image pick-up device, for example, a solid-state image pick-up device composed of pixels for storing an incident light from, e.g., an object as signal charges and a charge transfer section constituted with CCDs for sequentially transferring signal charges from these pixels in a vertical direction or in a horizontal direction is generally of a structure permitting signal charges stored at respective pixels of the solid-state image pick-up device to be swept away into the overflow drain region or the semiconductor substrate side by application of a shutter pulse.
In a solid-state image pick-up device of the field readout type in conformity with, e.g., the NTSC system, charge storage time at pixels within the field period is adjusted by application of the shutter pulse, thereby permitting the exposure time to be adjustable. Control of the exposure time is ordinarily called an electronic shutter function.
The control of the exposure time by the shutter pulse is carried out in units of 1H (horizontal scanning period) at least at the vertical scanning period in the case of the field readout type as described above. The reason why application timing of the shutter pulse is limited within the horizontal scanning period is to avoid noise being mixed into an image pick-up signal within the vertical scanning period.
However, since there is no need to fear that any noise will be mixed into an image pick-up signal within the vertical blanking period, control is carried out in time units considerably shorter than 1H.
Among video cameras using such solid-state image pick-up devices, there are some video cameras in which electronic shutter function is utilized for the iris control. Generally, a video camera comprises, as shown in FIG. 1, an imaging section composed of a lens 1 and a solid-state image pick-up device 2, a sample-hold circuit (hereinafter referred to as a S/H circuit) 3 for taking out an image pick-up signal S from an output from the solid-state image pick-up device 2 in the imaging section, an AGC (Automatic Gain Control) circuit 4, a gamma correction circuit 5, and a signal processing section composed of a white clipping circuit 6 and a black clipping circuit 7.
A video camera as mentioned above conventionally includes therein an exposure control circuit utilizing the electronic shutter function for the iris (stop) control. This exposure control circuit comprises, as shown in FIG. 1, a low-pass filter 8 for integrating image pick-up signals S from the S/H circuit 3, and a shutter control circuit 9.
More particularly, the low-pass filter 8 serves to detect the overall (average) brightness of picture (pictorial image of an object) by integrating the image pick-up signals S from the S/H circuit 3. The shutter control circuit 9 is basically constructed as a circuit for applying a shutter pulse signal to solid-state image pick-up device 2 in a manner to allow it to be in synchronism with a horizontal synchronizing signal within the vertical scanning period in a vertical synchronizing signal, and particularly serves to control the time from occurrence time point of the last shutter pulse up to occurrence time point of a subsequent readout pulse, i.e., exposure time by setting the number of output pulses of the shutter pulse signal applied to the solid-state image pick-up device 2 in accordance with the signal level of an output signal from the low-pass filter 8.
The shutter control circuit 9 performs, e.g., a function such that in the case where the signal level of an output signal from low-pass filter 8 is higher than reference level (i.e., an incident light is bright), the shutter speed is caused to be higher (the exposure time is shortened), while in the case opposite to the above (i.e., an incident light is dark), the shutter speed is caused to be lower (the exposure time is prolonged).
In this case, the above-mentioned readout pulse is a pulse signal for reading out signal charges stored at respective pixels of the solid-state image pick-up device 2. This readout pulse is outputted in the state superimposed on, e.g., pulse of the first phase of vertical transfer pulse signal of polyphase for vertically transferring signal charges stored at respective pixels.
It should be noted that there is employed in recent years a technique for permitting a shutter pulse signal having a period shorter than 1H to be generated at the vertical blanking period. The reason why such a technique is employed is to permit the brightness to be more finely adjusted so that the dynamic range of the exposure (iris) control is expanded.
In such conventional exposure control circuit, an exposure (iris) control is carried out such that when the signal level of an output signal from low-pass filter 8 is higher than reference level, the exposure time is shortened by one horizontal period (1H) every field so that the signal level of the output signal from low-pass filter 8 reaches the reference level, while when the signal level of an output signal from low-pass filter 8 is lower than the reference level, the exposure time is prolonged by one horizontal period (1H) every field so that the signal level of the output signal from low-pass filter 8 reaches the reference level.
Namely, the conventional exposure control circuit carries out an exposure control to recognize from an image pick-up signal S delivered from S/H circuit 3, whether a picture (pictorial image of an object) is bright, optimum or dark. It then adjusts the shutter speed to a higher level when the picture is bright, adjusts the shutter speed to be a lower level when the picture is dark and terminates the control operation when the current brightness is optimum.
However, in the conventional exposure control circuit, there is the following problem.
Namely, in the case where there takes place a momentary luminance change when a pedestrian passes in front of the video camera, or the like, the iris immediately responds thereto (i.e., exposure time immediately changes), so the luminance level of picture changes. As a result, the picture becomes very difficult to see. On the other hand, in the case of the eye of the human being, there is no possibility that brightness of the entirety changes even if a momentary luminance change as described above takes place.
To solve the above-mentioned problem, the following measure was taken in the related art in the exposure control circuit shown in FIG. 1, for example. Namely, the time constant of low-pass filter 8 for carrying out luminance detection is prolonged to allow change of luminance level to be dull to cause the change between three states (iris response) to be gentle even if a sudden change of luminance level takes place.
However, in this method, there takes place new problem that the iris response becomes dull even if a great change of luminance level which is not momentary takes place, thereby restricting the response speed.